Avalanche Forecast
Issued: Feb 24th, 2018 10:55AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
Travel in avalanche terrain is not recommended as very dangerous avalanche conditions exist. Expect widespread avalanches big enough to bury or kill you. Only travel in the backcountry today if you are certain that you can avoid avalanche terrain.
Summary
Detailed Forecast
Expect widespread avalanches in the new snow including many natural avalanches. Many of these could be big enough to kill you. The most dangerous areas will be steep slopes where new snow and wind have deposited 2 feet or more of cohesive snow since Friday. Select appropriate terrain to stay safe. Stay out of open slopes steeper than 35 degrees. Be aware of areas where avalanches commonly run. Avoid traveling below avalanche terrain and stay out of large avalanche paths. Only go into the backcountry today if you are sure of your ability to avoid avalanche terrain.
With several overlapping avalanche problems this will be a challenging time to travel in the mountains. While Storm and Wind Slabs exist in the upper snowpack, very dangerous Deep Slabs lurk below. It’s possible that avalanches in the upper snowpack could step down to deeper layers, triggering a very large and destructive avalanche. Deep Slab avalanches are very difficult to predict and give little warning sign. While they may be difficult to trigger your chances of surviving one are slim. If you dig 2-3 feet below the snow surface, you will find a thin layer of sugar-like facets adjacent to a hard crust. You can dig snowpits and use snowpack tests to confirm the strong over weak snow layers of the Deep Slab avalanche problem. Your best way to stay safe from these avalanches is to avoid triggering avalanches in the upper snowpack. Excellent travel conditions can still be found on slopes less than 30 degrees that are not connected to steep overhead slopes.
Snowpack Discussion
Heavy snowfall and strong winds have elevated avalanche danger, once again. Up to 1.5 feet of snow accumulated across the West Slopes by Saturday evening. Another 1-2 feet of snow will fall with strong winds by late Sunday. Snowfall starting at the end of the work week fell on a variety of surfaces including old snowflakes, surface hoar, and possibly some small facets.
Widespread natural avalanches were reported from the Mt Baker and Snoqualmie pass areas on Saturday. These were soft slab avalanches and some loose avalanches in the recent snow 1-1.5 feet deep with a few reported as big enough to bury a person. Some of these avalanches ran within the new snow while other slid on the interfce of new and old snow. Observers have not consistently found problematic weak layers at the February 23rd interface.
Last weekend, observers reported a widespread cycle of large to very large avalanches occurring with a potent storm. Avalanches ran in the storm snow as well as on facets, buried on the 13th of February. In many locations these weak sugar-like facets sit 3 feet below the surface on or adjacent to a hard crust layer. Avalanches, collapses, and whumphs have been triggered on this persistent weak layer well after last weekend’s initial avalanche cycle. Snowpack test and observations from around the region continue to demonstrate that this layer can fail and produce avalanches. Tests and profiles show the most consistent results from the Stevens and Snoqualmie pass areas. In the Mount Baker area the weak layer is more intermittent and can often show up as facets around a series of crusts.
A thin rain crust formed and was buried on 2/17 near and below 4000’ in the Snoqualmie area. Limited information about this snowpack layer has been reported other than it is ski-supportable.
Below the 2/8 crust there are no significant layers of concern.
Observations
Stevens and Snoqualmie Pass
On Saturday, Alpental Patrol reported that slab avalanches were easily triggered in the recent snow with ski cuts during control work. An NWAC observer reported many similar natural avalanches in the adjacent backountry at Snoqualmie Pass.
Also on Saturday, NWAC staff reported two different locations near Snoqualmie pass. While the 2/13 facet layer is showing rounding, test results indicated that the potential for triggering an avalanche still exists. This supports numerous similar snowpack tests results Stevens and Snoqualmie passes, the Crystal and Mt Rainier backcountry and to some extent the Mt Baker backcountry all conducted over the last week.
On Tuesday NWAC staff traveled through popular backcountry terrain near the Skyline area of Stevens Pass, finding the 2/13 facets on most slopes.
The most recent Persistent Slab avalanche was triggered on Monday at Stevens Pass backcountry (Stevens Pass-Hollywood Bowl-2-19). It failed on facets just above the 2/5 crust.
South
On Friday NPS rangers in the Paradise area reported 3-4 feet of snow over weak facets. No wind transported snow was observed.
NWAC Forecaster Dallas Glass observed a large audible whumph caused by a collapsing weak layer Tuesday near Paradise. Snowpack tests in the area indicated the persistent weak layer could fail and produce avalanches up to 4 feet deep.
No recent observations have been received from the Crystal backcountry. A higher level of uncertainty exists in this area.
Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 25th, 2018 10:55AM
